Wind turbine blade with cross-sectional sensors

ABSTRACT

A wind turbine blade extending along a longitudinal axis from a root end to a tip end and in a transverse plane perpendicular to the longitudinal axis, the transverse plane having a main axis extending through an elastic center point, wherein the wind turbine blade comprises a sensor system including a first sensor set for measuring a first bending moment in a first sensor position at a first distance from the root end, the first sensor set comprising a first primary sensor for measuring a primary component and a first secondary sensor for measuring a secondary component, wherein a first primary sensor axis in the transverse plane is oriented in a direction defined by the first primary sensor and the elastic center point, and a first secondary sensor axis in the transverse plane is oriented in a direction defined by the first secondary sensor and the elastic center point, and wherein an angle between the first primary sensor axis and the first secondary sensor axis is in the range from 50° to 130°

TECHNICAL FIELD

The present invention relates to a wind turbine blade, and a method andapparatus for determining or estimating loads such as bending moment ofa wind turbine blade, in particular root moment near or at the root endof a wind turbine blade.

BACKGROUND

Wind turbine manufacturers are constantly making efforts to improve theefficiency of their wind turbines in order to maximise the annual energyproduction. Further, the wind turbine manufacturers are interested inprolonging the lifetime of their wind turbine models, since it takes along time and a lot of resources to develop a new wind turbine model.Systems for monitoring operating parameters of a wind turbine and itscomponents such as the wind turbine blades have become an area ofincreased attention in order to optimize performance and prolonging thelifetime of different components.

Accordingly, there is a need for a wind turbine blade, methods andapparatus enabling accurate and efficient monitoring of one or moreoperating parameters of a wind turbine or wind turbine blade. An area ofparticular interest may be monitoring the loads and stresses applied ona wind turbine blade during operation of a wind turbine.

DISCLOSURE OF THE INVENTION

According to a first aspect, the invention provides a wind turbine bladeextending along a longitudinal axis from a root end to a tip end and ina transverse plane perpendicular to the longitudinal axis, thetransverse plane having a main axis extending through an elastic centerpoint, the wind turbine blade comprising a blade shell forming aprofiled contour including a pressure side and a suction side, as wellas a leading edge and a trailing edge with a chord having a chord lengthextending there between, the main axis being parallel to the chord, andwherein the profiled contour comprises a root region, an airfoil regionand optionally a transition region. The wind turbine blade may include asensor system comprising a first sensor set, e.g. for measuring a firstbending moment, in a first sensor position at a first distance from theroot end, the first sensor set comprising a first primary sensor formeasuring a primary component and a first secondary sensor for measuringa secondary component, the first sensor set being arranged in thetransition region or the airfoil region of the blade, wherein a firstprimary sensor axis in the transverse plane is oriented in a directiondefined by the first primary sensor and the elastic center point, and afirst secondary sensor axis in the transverse plane is oriented in adirection defined by the first secondary sensor and the elastic centerpoint. An angle between the first primary sensor axis and the firstsecondary sensor axis may be in the range from 50° to 130°.

By arranging the sensors in the transition region or the airfoil, thesensor readings are not encumbered with the non-linearities that areinherent with arranging the sensors in the root region of the blade dueto the connection with the hub of the wind turbine blade. Instead themoments at the root may be calculated via approximation functions and/orcurve fitting. Further, by arranging the sensor so that theaforementioned sensor axes forms an angle in an interval around 90degrees it is possible to maximize the sensitivity of the readings onthe primary and secondary bending moments.

According to a second and broader aspect, the invention provides a windturbine blade extending along a longitudinal axis from a root end to atip end and in a transverse plane perpendicular to the longitudinalaxis, the transverse plane having a main axis extending through anelastic center point, the wind turbine blade comprising a blade shellforming a profiled contour including a pressure side and a suction side,as well as a leading edge and a trailing edge with a chord having achord length extending there between, the main axis being parallel tothe chord. The wind turbine blade may include a sensor system comprisinga first sensor set, e.g. for measuring a first bending moment, in afirst sensor position at a first distance from the root end, the firstsensor set comprising a first primary sensor for measuring a primarycomponent and a first secondary sensor for measuring a secondarycomponent, wherein a first primary sensor axis in the transverse planeis oriented in a direction defined by the first primary sensor and theelastic center point, and a first secondary sensor axis in thetransverse plane is oriented in a direction defined by the firstsecondary sensor and the elastic center point. An angle between thefirst primary sensor axis and the first secondary sensor axis may be inthe range from 50° to 130°.

Accordingly, the wind turbine blade according to the invention allowsdetermination of bending moment with only two sensors in a cross-sectionsaving manufacture costs.

In the following, advantageous embodiments relating to both the firstand the second aspects are discussed.

Also disclosed is a wind turbine blade extending a long a longitudinalaxis from a root end to a tip end and in a transverse planeperpendicular to the longitudinal axis, the transverse plane having amain axis extending through an elastic center point, the wind turbineblade comprising a blade shell forming a profiled contour including apressure side and a suction side, as well as a leading edge and atrailing edge with a chord having a chord length extending therebetween, the main axis being parallel to the chord is provided. The windturbine blade may comprise a sensor system including a plurality ofsensor sets, each sensor set comprising a plurality of sensors includinga primary sensor and a secondary sensor for measuring a primarycomponent and a secondary component, respectively. The plurality ofsensor sets includes a first sensor set for measuring a first bendingmoment in a first sensor position at a first distance from the root end,and a second sensor set for measuring a second bending moment in asecond sensor position different from the first sensor position at asecond distance from the root end. The first distance may be at least 1m. The second distance may be at least 3 m.

Also disclosed is a method for estimating bending moment of a windturbine blade, e.g. of a wind turbine blade as disclosed herein. Thewind turbine blade extends along a longitudinal axis from a root end toa tip end and having a root region, a transition region and an airfoilregion, the method comprising the steps of obtaining a first sensor setsignal indicative of a first bending moment at a first sensor positiondifferent from the tip end along the longitudinal axis of the windturbine blade, and estimating a bending moment at a first estimationposition along the longitudinal axis based on the first sensor setsignal, wherein the first sensor position is different from the firstestimation position along the longitudinal axis.

Further, a blade monitoring system for monitoring a wind turbine bladecomprising a sensor system is disclosed, the blade monitoring systemcomprising a processing unit and an interface connected to theprocessing unit. The processing unit is configured for receiving a firstsensor set signal indicative of a first bending moment at a first sensorposition of a wind turbine blade extending along a longitudinal axisfrom a root end to a tip end. The processing unit is configured forestimating a bending moment at a first estimation position along thelongitudinal axis based on the first sensor set signal, wherein thefirst sensor position is different from the first estimation positionalong the longitudinal axis.

There is a need for a wind turbine blade having an optical sensor systemwith low loss.

Accordingly, a wind turbine blade is provided, the wind turbine bladeextending along a longitudinal axis from a root end to a tip end and ina transverse plane perpendicular to the longitudinal axis, thetransverse plane having a main axis extending through an elastic centerpoint, the wind turbine blade comprising a blade shell forming aprofiled contour including a pressure side and a suction side, as wellas a leading edge and a trailing edge with a chord having a chord lengthextending there between, the main axis being parallel to the chord. Thewind turbine blade comprises a sensor system with an optical pathcomprising a first optical fiber, a second optical fiber and optionallya patch optical fiber, the first optical fiber including a first corewith a first core diameter, wherein the first optical fiber extends froma first end to a second end and comprising at least one sensor, thesecond optical fiber including a second core with a second corediameter, wherein the second optical fiber extends from a first end to asecond end and comprising at least one sensor. The patch optical fiberincludes a patch core with a patch core diameter, wherein the patchoptical fiber extends from a first end to a second end. The patchoptical fiber may connect the first optical fiber and the second opticalfiber, and the first core diameter may be the same as the patch corediameter.

The sensor system with a patch optical fiber provides a high degree ofdesign freedom in designing a wind turbine blade and provides an easilyadaptable optical sensor system that may be used in different easilyconfigurable configurations and wind turbine blade models.

Further, manufacture of the wind turbine blade may be facilitated sinceassembly of the sensor system does not require specialist knowledge ortools.

Further, the wind turbine blade according to the invention allows foreasy reconfiguration of the sensor system after moulding and assembly ofblade shell parts.

Wind turbine comprising a plurality of wind turbine blades including afirst wind turbine blade as described herein, wherein the wind turbinecomprises a blade monitoring system configured to estimate bendingmoment of the first wind turbine blade based on sensor set signals fromthe sensor system of the first wind turbine blade.

The present invention relates to a wind turbine blade, e.g. for a rotorof a wind turbine having a substantially horizontal rotor shaft, therotor comprising a hub, from which the blade extends substantially in aradial direction when mounted to the hub, the blade extending along alongitudinal axis from a root end to a tip end and in a transverse planeperpendicular to the longitudinal axis, the transverse plane having amain axis extending through an elastic center point. The wind turbineblade comprises a blade shell forming a profiled contour including apressure side and a suction side, as well as a leading edge and atrailing edge with a chord having a chord length extending therebetween. The profiled contour, when being impacted by incident airflow,generates a lift. The profiled contour is divided into: a root regionhaving a substantially circular or elliptical profile closest to theroot end with a root diameter being the chord length at the root end, anairfoil region having a lift-generating profile furthest away from theroot end, and a transition region between the root region and theairfoil region, the transition region having a profile graduallychanging in the radial direction from the circular or elliptical profileof the root region to the lift-generating profile of the airfoil region,and with a shoulder having a shoulder width and a shoulder distance andlocated at the boundary between the transition region and the airfoilregion, wherein the blade has a blade length. By shoulder is meant theposition at which the wind turbine blade has its largest chord lengthand the shoulder distance is the distance from the root end to theshoulder. The length interval is defined from the root end to the tipend, the root end thus being positioned at r=0 and the tip end beingpositioned at r=L along the longitudinal axis.

The blade may comprise a blade shell with a shell body. The shell bodymay for instance be assembled from a pressure side shell and a suctionside shell, which are adhered or bonded to each other near the leadingedge and near the trailing edge. In another embodiment, the shell ismanufactured via a one-shot process, e.g. via a closed, hollow mouldingmethod.

The shell body may comprise a longitudinally extending load carryingstructure, such as a main laminate. Such a load carrying structure ormain laminate is typically formed as a fibre insertion which comprises aplurality of fibre reinforcement layers, e.g. between 20 and 50 layers.On each side of the load carrying structure, the blade typicallycomprises a sandwich structure with a core material, such as balsa woodor foamed polymer, and with an inner and outer skin made of fibrereinforced polymer.

One or more sensors may be arranged in the main laminate or in edgesthereof, e.g. the first sensor axis of one or more sensor sets may crossthe main laminate or a main laminate edge.

The blade shell is typically made of a fibre reinforced polymermaterial. The reinforcement fibres may for instance be glass fibres,carbon fibres, aramid fibres, metallic fibres, such as steel fibres, orplant fibres, whereas the polymer for instance may be epoxy, polyesteror vinylester.

The wind turbine blade comprises a sensor set comprising at least onesensor, e.g for measuring or determining bending moments. A sensor setmay alternatively or in combination be configured for measuring otherparameters. The at least one sensor set includes a first sensor setpositioned at a first sensor position along the longitudinal axis. Thefirst sensor set may be positioned at a first distance d₁ from the rootend.

The wind turbine blade may comprise a plurality of sensors including thefirst sensor set and a second sensor set positioned at a second sensorposition along the longitudinal axis. The second sensor set may bepositioned at a second distance d₂ from the root end.

The plurality of sensor sets may include a third sensor set formeasuring a third bending moment in a third sensor position at a thirddistance d₃ from the root end.

The plurality of sensor sets may include a fourth sensor set formeasuring a fourth bending moment in a fourth sensor position at afourth distance d₄ from the root end.

The plurality of sensor sets may include a fifth sensor set formeasuring a fifth bending moment in a fifth sensor position at a fifthdistance d₅ from the root end.

The first distance d₁ may be in the range from about 1 m to about 20 mand the second distance d₂ may be in the range from about 3 m to about50 m.

Optionally, sensor position distances from the root end may depend onthe length L of the wind turbine blade and/or the shoulder distance orposition d_(s).

The first distance d₁ may be selected in the range from d_(1,min) tod_(1,max). The first distance d₁ may be in the range from about 4 m toabout 15 m such as in the range from about 6 m to about 10 m, preferablyabout 8 m.

The first distance d₁ may depend on the shoulder distance d_(s), i.e. d₁may be a function of the shoulder distance d_(s). For example, d₁ may begiven by

d ₁=α₁ d _(s),

where α₁ is in the range from 0.2 to 1.0.

The first distance d₁ may depend on the length of the wind turbineblade, i.e. d₁ may be a function of the blade length L. For example, d₁may be given by

d ₁=β₁ L,

where β₁ is in the range from about 0.05 to about 0.95.]

The first distance d₁ may depend on the root diameter d_(root), i.e. d₁may be a function of the root diameter d_(root). The first distance d₁may be at least a diameter of the root.

For example, d₁ may be given by

d ₁γ₁ d _(root),

where γ₁ is at least 0.8.

The minimum first distance d_(1,min) may depend on the root diameterd_(root), e.g. be given by

d _(1,min)=γ₁ d _(root),

where γ₁ is at least 0.8, such as about 1.

The maximum first distance d_(1,max) may depend on the shoulder distanced_(s), e.g. be given by

d _(1,max)=α₁ d _(s),

where α₁ is in the range from 0.5 to 1.0.

The shoulder distances d_(s) may be in the range from 11 m to 15 m, e.g.about 13 m for a wind turbine blade having a length about 61.5 m.

The root diameter d_(root) may be in the range from 2 m to 5 m, e.g.about 2.5 m for a wind turbine blade having a length from 40 m-50 m, orabout 3.5 m for a wind turbine blade having a length from 60 m-75 m.

The second distance d₂ may be selected in the range from d_(2,min) tod_(2,max). The second distance d₂ may be in the range from about 5 m toabout 40 m, such as in the range from about 10 m to about 30 m such asabout 12 m or about 23 m.

The second distance d₂ may depend on the shoulder distance d_(s), i.e.d₂ may be a function of the shoulder distance d_(s). For example, d₂ maybe given by

d ₂=α₂ d _(s),

where α₂ is in the range from 0.5 to 10.0.

The second distance (d₂) may depend on the length of the wind turbineblade, i.e. d₂ may be a function of the blade length L. For example, d₂may be given by

d ₂=β₂ L,

where β₂ is in the range from 0.1 to 0.8.

The second distance d₂ may depend on the root diameter d_(root), i.e. d₂may be a function of the root diameter d_(root). For example, d₂ may begiven by

d ₂=γ₂ d _(root),

where γ₂ is at least 0.8.

The minimum second distance d_(2,min) may depend on the root diameterd_(root), e.g. be given by

d _(2,min)=γ₂ d _(root),

where γ₂ is at least 0.8, such as about 2.

The maximum second distance d_(2,max) may depend on the shoulderdistance d_(s), e.g. be given by

d _(2,max)=α₂ d _(s),

where α₂ is in the range from 0.5 to 3.0.

The distances d₁, d₂, d₃, . . . of sensor sets from the root end may beat least 6 m in order to minimize or avoid undesired non-linearitiesfrom pitch bearings in sensor measurements.

In one or more embodiments, the distances d₁, d₂, d₃, . . . of sensorsets from the root end may be less than 25 m, e.g. less than 20 m, toreduce or avoid excessive errors of superposition in sensormeasurements.

The distances between sensor sets along the longitudinal axis areselected in order to facilitate desired measurements, e.g. forestimating bending moments along the wind turbine blade such as in oneor more estimation positions. Distances between sensor positions areindicated as d_(ij), where i and j are index numbers for sensor sets andsensors thereof.

The distance d₁₂ between the first sensor set and optional second sensorset along the longitudinal axis may be in the range from about 1 m toabout 30 m, such as from about 3 m to about 20 m, e.g. about 4 m, about10 m, about 15 m.

The distance d₁₃ between the first sensor set and optional third sensorset along the longitudinal axis may be in the range from about 1 m toabout 50 m, such as from about 10 m to about 40 m, e.g. about 15 m,about 25 m, about 35 m.

The distance d₁₄ between the first sensor set and optional fourth sensorset along the longitudinal axis may be in the range from about 1 m toabout 60 m, such as from about 15 m to about 50 m, e.g. about 20 m,about 30 m, about 40 m.

The distance d₁₅ between the first sensor set and optional fifth sensorset along the longitudinal axis may be in the range from about 20 m toabout L−d₁, such as from about 20 m to about 60 m, e.g. about 30 m,about 40 m, about 50 m.

A sensor set may comprise one or more sensors. The one or more sensorsof a sensor set may include a primary sensor and optionally a secondarysensor. Sensor(s) of a sensor set may be adapted to measure bendingmoment components, i.e. a sensor set may comprise a primary sensor formeasuring a primary bending moment component M_(X) about a first axisperpendicular to the longitudinal axis, and/or a secondary sensor formeasuring a secondary bending moment component M_(Y) about a second axisperpendicular to the longitudinal axis. In one or more embodiments, asensor set consists of two sensors for measuring bending momentcomponents, thereby allowing bending moment measurements on the windturbine blade with only two sensors.

The wind turbine blade comprises a first sensor set, e.g. for measuringa first bending moment, in a first sensor position at a first distancefrom the root end.

The first sensor set may comprise a first primary sensor for measuring aprimary component of the first bending moment (M_(X,1)) about a firstaxis perpendicular to the longitudinal axis (first primary sensor) and afirst secondary sensor for measuring a secondary component of the firstbending moment (M_(Y,1)) about a second axis perpendicular to thelongitudinal axis (first secondary sensor).

The second sensor set may comprise a second primary sensor for measuringa primary component (M_(X,2)) of the second bending moment about a firstaxis perpendicular to the longitudinal axis (second primary sensor) anda second secondary sensor for measuring a secondary component of thesecond bending moment (M_(Y,2)) about a second axis perpendicular to thelongitudinal axis (second secondary sensor).

Sensors of a sensor set are positioned at the same distance from theroot end, i.e. in the same transverse plane. In one or more embodiments,sensors of a sensor set, e.g. a primary sensor and a secondary sensor,may be displaced along the longitudinal axis. The distance betweensensors of a sensor set along the longitudinal axis should be as smallas possible. The maximum distance between sensors of a sensor set may beless than 1 m, such as less than 0.5 m. Larger distances may beemployed. In case of displaced sensors of a sensor set, the sensor setdistance to the root is the mean distance for sensors of the sensor set.

The length L of the wind turbine blade may be at least 40 m.

Root moments of a wind turbine blade are desired from a controlperspective of the wind turbine, e.g. in order to control pitch andother operating parameters in order to optimize operation and poweroutput of the wind turbine. However, measurement of the root moment withsensors positioned at the root end of the blade is largely affected bynon-linear moment contributions from the pitch bearing.

Correct positioning of sensors in a wind turbine blade is important inorder to obtain a precise measurement and in order to reduce oreliminate undesired effects such as nonlinear effects. It is from a costand manufacture perspective desired to employ a low number of sensors.

A primary sensor is positioned on a primary sensor axis extendingthrough the elastic center in the transverse plane with the primarysensor. A secondary sensor is positioned on a secondary sensor axisextending through the elastic center in the transverse plane with thesecondary sensor.

The angle between the first primary sensor axis and the first secondarysensor axis may be in the range from about 85° to about 95°. The anglebetween the first primary sensor axis and the first secondary sensoraxis may be about 90°.

In case of perpendicular first and second sensor axes, the first bendingmoment M₁ may be given as:

M ₁√{square root over (M _(X,1) ² +M _(Y,1) ²)}

An estimated primary component of the bending moment at the firstestimation position M_(X,est,1), e.g. at the root end, may be given as afunction of one or more sensor signals S₁₁, S₁₂, S₂₁, S₂₂, . . . , whereS₁₁ is the first primary sensor signal, S₁₂ is the first secondarysensor signal, S₂₁ is the second primary sensor signal, S₂₂ is thesecond secondary sensor signal, etc.

An estimated secondary component of the bending moment at the firstestimation position M_(Y,est,1), e.g. at the root end, may be given as afunction of one or more sensor signals S₁₁, S₁₂, S₂₁, S₂₂, . . . , whereS₁₁ is the first primary sensor signal, S₁₂ is the first secondarysensor signal, S₂₁ is the second primary sensor signal, S₂₂ is thesecond secondary sensor signal, etc.

Sensor signals may be multiplexed in time and/or infrequency/wavelength. Time multiplexing may be preferred for reducingthe number of components in the reading unit and reduce the costs of thesensing system.

The estimated primary component of the bending moment at the firstestimation position M_(x,est,1) may be estimated based only on primarysensor signals from primary sensors of one or more sensor sets. Theestimated secondary component of the bending moment at the firstestimation position M_(Y,est,1) may be estimated based only on secondarysensor signals from secondary sensors of one or more sensor sets.Additionally or alternatively, M_(x,est,1) may be estimated based onsecondary sensor signals from secondary sensors of one or more sensorsets and/or M_(Y,est,1) may be estimated based on primary sensor signalsfrom primary sensors of one or more sensor sets.

The bending moment M_(est,1) at a first estimation position may beestimated or given as:

M _(est,1)=√{square root over (M _(X,est,1) ² +M _(Y,est,1) ²)}.

The bending moment M_(est,1) at a first estimation position may beestimated or given as:

M _(est,1)=√{square root over (α_(X) M _(X,est,1) ²+β_(Y) M _(Y,est,1)²)},

where α_(X) and β_(Y) are compensation factors for compensating fornon-perpendicular first axis and second axis.

The wind turbine blade may comprise a second sensor set, e.g. formeasuring a second bending moment, in a second sensor position at asecond distance from the root end. The sensor set may comprise a secondprimary sensor for measuring a primary component and a second secondarysensor for measuring a secondary component, wherein a second primarysensor axis in the transverse plane is oriented in a direction definedby the second primary sensor and the elastic center point, and a secondsecondary sensor axis in the transverse plane is oriented in a directiondefined by the second secondary sensor and the elastic center point. Anangle between the second primary sensor axis and the second secondarysensor axis may be in the range from 50° to 130°, such as in the rangefrom 85° to 95°. The angle between the first primary sensor axis and thefirst secondary sensor axis may be about 90°.

The primary sensor(s), e.g. the first primary sensor and/or the secondprimary sensor, may for instance be arranged at the pressure side or thesuction side of the blade, advantageously at or embedded in a loadcarrying structure such as a main laminate of the blade shell structure.

The secondary sensor(s), e.g. the first secondary sensor and/or thesecond secondary sensor, may for instance be arranged at the leadingedge or the trailing edge of the wind turbine blade

Positioning primary and secondary sensors on the pressure side and atthe leading edge, respectively, may facilitate employment of straingauge sensors. One or more of the sensors, such as the primary and/orthe secondary sensors, may be strain gauge sensors.

One or more of the sensors, such as the primary and/or the secondarysensors, may be optical sensors, such as fiber Bragg gratings.

At least one of the sensors may be embedded in the blade shell. One ormore sensors may be attached to or mounted on the inner surface of theblade shell. The wind turbine blade may comprise a beam attached to theshell and at least one of the sensors may in this case be mounted on thebeam. One or more sensors may be mounted on or embedded in the mainlaminate of a shell body.

The wind turbine blade may comprise a web attached to the shell and atleast one of the sensors may be mounted on the web.

The one or more sensor sets, e.g. the first sensor set, and/or thesecond sensor set may be arranged in the root region, the transitionregion or the airfoil region of the wind turbine blade.

The sensors of the wind turbine blade provide sensor signals indicativeof a bending moment, e.g. in the form of strain/pressure signalsindicating strain/pressure applied to the sensor. The strain/pressuresignals of sensors can be transformed to bending moments or componentsthereof by use of sensor systems parameters, e.g. determined duringcalibration and/or design of the wind turbine blade.

A sensor signal indicative of a bending moment may be an optical signalfrom an optical sensor, e.g. a fiber Bragg grating, in an optical fiber,where the sensor signal is reflected light having a wavelength dependingon the strain applied to the sensor. The sensor signal may be fed to areading unit positioned e.g. in the wind turbine blade or in the hub ofa wind turbine. The reading unit may be configured to determine orderive the wavelength of one or more sensor signals and provide thewavelength(s) of sensor signal(s) to a processing unit in a blademonitoring system. The reading unit may be a separate device connectableto a sensor system and a blade monitoring system for receiving sensorsignals and forwarding processed sensor signals to the blade monitoringsystem. In one or more embodiments, the reading unit may be embedded inthe blade monitoring system, i.e. the blade monitoring system comprisesthe reading unit and is connectable to the sensor system via one or moresensor ports of the interface.

In one or more embodiments where the sensors are implemented as fiberBragg gratings in an optical fiber, the sensor signals are opticalsignals with wavelengths λ₁₁ (first primary sensor), λ₁₂ (firstsecondary sensor), λ₂₁, (second primary sensor), λ₂₂ (second secondarysensor), . . . , λ_(ij) where i and j are index numbers for sensor setsand sensors thereof. The sensor signals may be multiplexed in timeand/or frequency/wavelength. The strain and/or pressure applied to thesensors may be derived from the wavelengths of the sensor signals. Theprocessing unit may be configured to derive bending moments and/orcomponents thereof from the sensor signals, e.g. based on sensor systemparameters stored in a memory unit.

For a wind turbine with an optical sensor system including a secondoptical fiber and a patch optical fiber, the second core diameter may bethe same. The optical path in the sensor system may have the samediameter, i.e. different optical fibers in the system may have the samecore diameter or mode field diameter.

The first optical fiber may comprise a first primary sensor andoptionally a second primary sensor for indicating strain at a firstdistance and optionally at a second distance, respectively, from theroot end of the wind turbine blade.

The second optical fiber may comprise a first secondary sensor andoptionally a second secondary sensor for indicating strain at a firstdistance and optionally at a second distance, respectively, from theroot end of the wind turbine blade.

Standard patch optical fibers have a core diameter of 9 μm. A sensingfiber typically has a core diameter of less than 7 μm. Having the samecore diameter in the first optical fiber and the patch optical fiberprovides an optical path with low damping and thus a sensor system withlow loss is provided. An optical sensor system with low loss, e.g. lessthan 3.5 dB, may be provided in order to meet sensor system requirementsfrom the reading unit or in order to reduce the requirements to thereading unit.

The sensor system may comprise a plurality of patch optical fibers withthe same patch core diameter.

The sensor system may comprise a number of optical fibers, each opticalfiber comprising one or more sensors, including a third optical fiberand/or a fourth optical fiber. The third optical fiber may be positionedparallel and adjacent to the first optical fiber in the wind turbineblade and/or the fourth optical fiber may be positioned parallel andadjacent to the second optical fiber in the wind turbine blade. Morethan two optical fibers with sensors may allow for easy repair of thesensor system. For example, if the first optical fiber breaks, the patchoptical fiber is simply connected to the third optical fiber and theblade monitoring system is configured to estimate bending moments basedon sensor signals from sensors in the second and third optical fiberinstead of estimating bending moments based on sensor signals fromsensors in the first and second optical fiber.

Provision of one or more patch cables with the same core diameters asthe optical fibers with sensor(s) facilitates a high degree of freedomfor the blade designer.

An optical fiber with a relatively small core diameter, e.g. less than 7μm, may be preferred for sensing in the wind turbine blade where a lowbend-induced loss may be of importance. A core diameter of 4.2 μm may beused since a high photosensitivity may be desirable in order tofacilitate fabrication of certain types of fiber Bragg gratings (FBGs).

Accordingly, the first core diameter, the second core diameter and/orthe patch core diameter may be less than 7 μm, such as 6.4 μm, 5.3 μm or4.2 μm.

The first and/or second optical fibers may be single mode fiber(s) witha design wavelength of 1550 nm.

The patch optical fiber(s) may be single mode fiber(s) with a designwavelength of 1550 nm.

In one or more embodiments, the first and second optical fibers and thepatch optical fiber are optical fibers having MFD of 6.4 μm and outerdiameter of 80 μm.

In one or more embodiments the first and second optical fibers and thepatch optical fiber are optical fibers having MFD of 4.2 μm and outerdiameter of 125 μm.

The optical fibers may have a cut-off wavelength in the range from 1350to 1500 nm.

The optical fibers may be SM1500(4.2/125) optical fibers.

The distance between sensors of the sensor system may be at least 3 malong the optical path of the sensor system. In particular, the distancebetween the first primary sensor and the second primary sensor may be atleast 3 m, e.g. at least 5 m, at least 8 m, along the first opticalfiber in order to enable sensing at distances along the longitudinalaxis of the wind turbine. The distance between the first secondarysensor and the second secondary sensor may be at least 3 m along thesecond optical fiber, e.g. at least 5 m, at least 8 m.

An inter-sensor distance of at least 3 m facilitates time multiplexingof sensor signals. The distances between sensors in an optical fiber areselected in order to arrange sensors in desired positions in the windturbine blade.

The first optical fiber and the second optical fiber may be mainly or atleast partly embedded in the blade shell. Preferably, one or more endsof the first optical fiber and/or the second optical fiber are providedwith connector parts for coupling or connection to a patch opticalfiber, a reading unit or another optical fiber with sensors. Theconnector part(s) may be embedded or accommodated in one or moreconnector boxes mounted on or moulded into the blade shell for allowingeasy installation of one or more patch optical fibers. Connector partsat ends of the optical fibers provide optical coupling of the fibercores of the optical fibers.

The first optical fiber and the second optical fiber may be at leastpartly adhered or mounted to the blade shell.

The wind turbine blade may comprise a reading unit comprising at leastone sensor port including a first sensor port for optically coupling thesensor system, e.g. the first optical fiber, to the reading unit. Thereading unit may be configured for reading sensor signals of opticalsensors in the sensor system as described above. The reading unit may beconfigured to derive a plurality of sensor signals indicative of strainon sensors of the sensor system in the wind turbine blade.

The reading unit may comprise a second sensor port, for example suchthat the second end of the second optical fiber can be optically coupledto the second sensor port for reading sensor signals of optical sensorsin the sensor system. This configuration in combination with opticalcoupling of the second end of the first optical fiber and the first endof the second optical fiber enables monitoring of sensors even in caseof a damage at a single point in the optical path formed by the firstoptical fiber and the second optical fiber.

The temperature may affect operation and characteristics of sensors,i.e. the sensor signals from the optical sensors may depend on thetemperature. A temperature-insensitive determination of bending momentmay be desired.

Accordingly, the sensor system may comprise a first temperature sensorin the first optical fiber and/or a second temperature sensor in thesecond optical fiber. A fiber Bragg grating (FBG) of an optical fibermay be arranged in the wind turbine blade such that no changes in strainor pressure are applied to the FBG. Thereby the FBG may function as atemperature sensor.

The first core diameter of the first optical fiber may be equal to thesecond core diameter of the second optical fiber.

The sensor system may comprise a beam splitting/combining unit having afirst, second and third port, wherein the first port is opticallycoupled to the first end of the first optical fiber and the second portis optically coupled to the second end of the second optical fiber, suchthat sensor signals from the first optical fiber are combined withsensor signals from the second optical fiber on the third port connectedto a reading unit.

The processing unit may be configured to compensate for temperaturevariation, i.e. estimation of bending moments may comprise applying acompensation factor to the sensor signals, the compensation factor beingbased on one or more temperature signals from the first temperaturesensor and/or the second temperature sensor.

Optical sensors are preferred in order to reduce or eliminate thedamages of lightning strike.

The first optical fiber may comprise a first end connector part and/or asecond end connector part for connecting the first end and second end,respectively, to a reading unit, a blade monitoring system and/or otheroptical fibers.

The second optical fiber may comprise a first end connector part and/ora second end connector part for connecting the first end and second end,respectively, to a reading unit, a blade monitoring system and/or otheroptical fibers.

The connector parts may be an E2000 connector.

In the method for estimating bending moment of a wind turbine blade,estimating a bending moment at a first estimation position may becarried out assuming a zero bending moment at the tip end of the windturbine blade.

The distance between the first sensor position and the first estimationposition along the longitudinal axis may be at least 1 m, such as atleast 3 m, preferably in the range from 3 m to about 12 m.

During operation of a wind turbine, information on loads on the windturbine blade may be of interest from a turbine control viewpoint.Accordingly, information on root moments of wind turbine blade may be adesired parameter. The first estimation position may be any positionalong the longitudinal axis, such as at the root end of the wind turbineblade.

The first bending moment may have a primary component about a first axisperpendicular to the longitudinal axis and/or a secondary componentabout a second axis perpendicular to the longitudinal axis.

The method may comprise obtaining a second sensor set signal indicativeof a second bending moment at a second sensor position along thelongitudinal axis and estimating a bending moment at a first estimationposition may be based on the second sensor set signal.

In the method, the distance between the first sensor position and thesecond sensor position along the longitudinal axis may be at least 1 m.

The second bending moment may have a primary component about a firstaxis perpendicular to the longitudinal axis and a secondary componentabout a second axis perpendicular to the longitudinal axis.

The first axis and the second axis may be perpendicular or form anangle, e.g. the smallest angle may be in the range from 75° to about90°.

The first axis may be perpendicular to the primary sensor axis. Thesecond axis may be perpendicular to the secondary sensor axis.

In the method and in the blade monitoring system, estimating a bendingmoment may comprise estimating a primary component M_(X,est) about afirst axis perpendicular to the longitudinal axis and/or a secondarycomponent M_(Y,est) about a second axis perpendicular to thelongitudinal axis at a first estimation position and/or at a secondestimation position along the longitudinal axis. Estimating a bendingmoment may comprise curve fitting.

In the method and in the blade monitoring system, the bending momentM_(est,1) or components thereof M_(X,est,1) and/or M_(Y,est,1) at thefirst estimation position may be estimated by using a firstapproximation function from the tip end to the first sensor position anda second approximation function from the first sensor position to thefirst estimation position. The second approximation function may bebased on the first approximation function. Different approximationfunctions may be employed for primary and secondary components, i.e. afirst and second primary approximation function may be employed for theprimary component and a first and second secondary approximationfunction may be employed for the secondary component

The first approximation function may be selected from a cubic splinefunction and a polynomial function. The polynomial function may be afirst order, a second order, a third order, a fourth or higher orderpolynomial function. The second approximation function may be a linearinterpolation.

The first sensor position may be located in the transition region or theairfoil region of the wind turbine blade.

The method may comprise transmitting the estimated bending moment orcomponents thereof to a control system of a wind turbine, e.g. a blademonitoring system, pitch control system, a wind turbine controller, awind park controller, alarm system or the like.

The blade monitoring system may be configured for implementing one ormore parts or steps of the method described herein. The blade monitoringsystem enables determination and estimation of bending moments orcomponents thereof applied to one or more wind turbine blades of a windturbine, e.g. during operation of the wind turbine. Thus, the inventionallows for a control system of the wind turbine to adjust operationalparameters such as pitch angles in order to optimize power productionand lifetime of the wind turbine and wind turbine blades.

The blade monitoring system may comprise a memory unit connected to theprocessing unit, the memory unit being configured for storing sensorsystem parameters of the sensor system. The processing unit may beconfigured for estimating a bending moment based on sensor systemparameters stored in the memory unit, e.g. sensor system parametersderived during calibration or manufacture of the wind turbine blade.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become readily apparent to those skilled in the art by thefollowing detailed description of exemplary embodiments thereof withreference to the attached drawings, in which:

FIG. 1 illustrates a wind turbine,

FIG. 2 illustrates a wind turbine blade,

FIG. 3 is a cross section of a wind turbine blade,

FIG. 4 illustrates different views of a wind turbine blade,

FIG. 5 is a flow diagram of an exemplary method according to theinvention,

FIG. 6 illustrates a cross section of a wind turbine blade,

FIG. 7 illustrates a cross section of a wind turbine blade,

FIG. 8 illustrates a cross section of a wind turbine blade,

FIG. 9 illustrates a cross section of a wind turbine blade,

FIG. 10 illustrates a cross section of a wind turbine blade,

FIG. 11 illustrates a wind turbine blade with sensor system according tothe invention,

FIG. 12 schematically illustrates a first optical fiber and a patchoptical fiber,

FIG. 13 illustrates a blade monitoring system,

FIG. 14 illustrates a blade monitoring system, and

FIG. 15 illustrates estimated bending moment with curve fitting.

DETAILED DESCRIPTION OF THE INVENTION

The figures are schematic and simplified for clarity, and they merelyshow details which are essential to the understanding of the invention,while other details have been left out. Throughout, the same referencenumerals are used for identical or corresponding parts.

FIG. 1 illustrates a conventional modern upwind wind turbine accordingto the so-called “Danish concept” with a tower 4, a nacelle 6 and arotor with a substantially horizontal rotor shaft. The rotor includes ahub 8 and three blades 10 extending radially from the hub 8, each havinga blade root 16 nearest the hub and a blade tip 14 furthest from the hub8. The rotor has a radius denoted R.

FIG. 2 shows a schematic view of a first embodiment of a wind turbineblade 10 according to the invention. The wind turbine blade 10 has theshape of a conventional wind turbine blade and comprises a root region30 closest to the hub, a profiled or an airfoil region 34 furthest awayfrom the hub and a transition region 32 between the root region 30 andthe airfoil region 34. The blade 10 comprises a leading edge 18 facingthe direct ion of rotation of the blade 10, when the blade is mounted onthe hub, and a trailing edge 20 facing the opposite direction of theleading edge 18.

The airfoil region 34 (also called the profiled region) has an ideal oralmost ideal blade shape with respect to generating lift, whereas theroot region 30 due to structural considerations has a substantiallycircular or elliptical cross-section, which for instance makes it easierand safer to mount the blade 10 to the hub. The diameter (or the chord)of the root region 30 may be constant along the entire root area 30. Thetransition region 32 has a transitional profile gradually changing fromthe circular or elliptical shape of the root region 30 to the airfoilprofile of the airfoil region 34. The chord length of the transitionregion 32 typically increases with increasing distance r from the hub.The airfoil region 34 has an airfoil profile with a chord extendingbetween the leading edge 18 and the trailing edge 20 of the blade 10.The width of the chord in the airfoil region decreases with increasingdistance r from the hub.

A shoulder 40 of the blade 10 is defined as the position, where theblade 10 has its largest chord length. The shoulder 40 is typicallyprovided at the boundary between the transition region 32 and theairfoil region 34.

The blade 10 has different airfoil profiles 41, 42, 43, 44, 45, 46 alongthe longitudinal axis of the blade.

As illustrated in FIG. 4, the wind turbine blade 10 comprises at leastone sensor set including a first sensor set positioned at a firstposition along the longitudinal axis.

The first sensor set comprises a first primary sensor 47A and optionallya first secondary sensor 47B positioned at a first distance d₁ from theroot end. The sensor 47A and the sensor 47B may be displaced a distanced_(1,12) along the longitudinal direction. The distance d_(1,12) may beless than 1 m.

Optionally, the wind turbine blade 10 comprises a second sensor setpositioned at a second position along the longitudinal axis. The secondsensor set comprises a second primary sensor 48A and optionally a secondsecondary sensor 48B positioned at a second distance d₂ from the rootend. The sensor 48A and the sensor 48B may be displaced a distanced_(2,12) along the longitudinal direction. The distance d_(2,12) may beless than 1 m.

In the wind turbine blade 10, the sensors are optical sensors in theform of an optical fiber with fiber Bragg gratings embedded in the shellof the wind turbine blade. The sensors of the wind turbine blade may bea part of the same optical fiber and/or be a part of different opticalfiber sections coupled by one or more optical connectors.

It should be noted that the chords of different sections of the bladenormally do not lie in a common plane, since the blade may be twistedand/or curved (i.e. pre-bent), thus providing the chord plane with acorrespondingly twisted and/or curved course, this being most often thecase in order to compensate for the local velocity of the blade beingdependent on the radius from the hub.

Table 1 below illustrates different suitable combinations of sensorpositions (distances from the root end), optionally dependent on thelength of the wind turbine blade.

TABLE 1 Sensor positions. d₁/m d₂*/m d₃*/m d₄*/m d₅*/m L/m 2-L  3-L 10-L20-L 30-L ≧40 2-20  3-40 10-45 4-15  5-30 4-10 10-30 6-10 10-15 15-20 510 8 12 23 40 50 ≧60 1.5 * d_(root) 0.9 * d_(s) 8 d₁ + d₁₂ 8 12 8 23 40*if present

Sensor position configuration may depend on the number of sensor setsavailable and estimation position(s). A sensor position near the rootend may be desirable, however a sensor position too near the root end isnot desirable due to contributions or noise from the pitch bearings.

FIGS. 3 and 4 depict parameters, which may be used to explain thegeometry of the wind turbine blade according to the invention.

FIG. 3 shows a schematic view of an airfoil profile 50 of a typicalblade of a wind turbine depicted with the various parameters, which aretypically used to define the geometrical shape of an airfoil. Theairfoil profile 50 has a pressure side 52 and a suction side 54, whichduring use—i.e. during rotation of the rotor—normally face towards thewindward (or upwind) side and the leeward (or downwind) side,respectively. The airfoil 50 has a chord 60 with a chord length cextending between a leading edge 56 and a trailing edge 58 of the blade.The airfoil 50 has a thickness t, which is defined as the distancebetween the pressure side 52 and the suction side 54. The thickness t ofthe airfoil varies along the chord 60. The deviation from a symmetricalprofile is given by a camber line 62, which is a median line through theairfoil profile 50. The median line can be found by drawing inscribedcircles from the leading edge 56 to the trailing edge 58. The medianline follows the centres of these inscribed circles and the deviation ordistance from the chord 60 is called the camber f. The asymmetry canalso be defined by use of parameters called the upper camber (or suctionside camber) and lower camber (or pressure side camber), which aredefined as the distances from the chord 60 and the suction side 54 andpressure side 52, respectively.

Airfoil profiles are often characterised by the following parameters:the chord length c, the maximum camber f, the position d_(f) of themaximum camber f, the maximum airfoil thickness t, which is the largestdiameter of the inscribed circles along the median camber line 62, theposition d_(t) of the maximum thickness t, and a nose radius (notshown). These parameters are typically defined as ratios to the chordlength c. Thus, a local relative blade thickness t/c is given as theratio between the local maximum thickness t and the local chord lengthc. Further, the position d_(p) of the maximum pressure side camber maybe used as a design parameter, and of course also the position of themaximum suction side camber.

FIG. 4 shows other geometric parameters of the blade. The blade has atotal blade length L. As shown in FIG. 3, the root end is located atposition r=0, and the tip end located at r=L. The shoulder 40 of theblade is located at a position r=d_(s), and has a shoulder width W,which equals the chord length at the shoulder 40. The diameter of theroot is defined as d_(root). The curvature of the trailing edge of theblade in the transition region may be defined by two parameters, viz. aminimum outer curvature radius r_(o) and a minimum inner curvatureradius r_(i), which are defined as the minimum curvature radius of thetrailing edge, seen from the outside (or behind the trailing edge), andthe minimum curvature radius, seen from the inside (or in front of thetrailing edge), respectively. Further, the blade is provided with apre-bend, which is defined as Δy, which corresponds to the out of planedeflection from a pitch axis 22 of the blade.

FIG. 5 illustrates an exemplary method according to the presentinvention. The method 100 comprises obtaining 102 a first sensor setsignal indicative of a first bending moment at a first sensor positionalong the longitudinal axis of the wind turbine blade. Further, themethod 100 comprises estimating 104 a bending moment at a firstestimation position along the longitudinal axis based on the firstsensor set signal, wherein the first sensor position is different fromthe first estimation position along the longitudinal axis. The methodmay be employed on a wind turbine blade as described herein. Optionally,the method 100 comprises obtaining 106 a second sensor set signalindicative of a second bending moment at a second sensor position alongthe longitudinal axis, and estimating 104 a bending moment is based onthe second sensor set signal. Sensor signals may be obtained seriallyand/or in parallel.

The first sensor set signal comprises a first primary sensor signal froma first primary sensor (47A) and a first secondary sensor signal from afirst secondary sensor (47B). The first primary sensor signal indicatesa primary component M_(X,1) of the first bending moment and the firstsecondary sensor signal indicates a secondary component M_(Y,1) of thefirst bending moment.

FIG. 6 and FIG. 7 are cross sections illustrating examples of sensor setpositioning on a wind turbine blade.

In FIG. 6, the wind turbine blade 10′ comprises a first sensor set witha first primary sensor 47A and a second primary sensor 47B at a firstdistance d₁ from the root. The first primary sensor 47A lies on a firstprimary sensor axis 74 extending through the elastic center 70 of theblade cross section (transverse plane at first distance d₁). The firstsecondary sensor 47B lies on a first secondary sensor axis 76 extendingthrough the elastic center 70 of the blade cross section. In FIG. 6, theelastic center lies on the chord 60 and thus the main axis 72 coincidewith the chord 60. The angle α₁ between the two sensor axes 74 and 76 is90°. The angle β₁ between the main axis 72 and the first primary sensoraxis 74 is 90°

In FIG. 7, the wind turbine blade 10″ comprises a first sensor set witha first primary sensor 47A and a second primary sensor 47B at a firstdistance d₁ from the root.

The first primary sensor 47A lies on a first primary sensor axis 74extending through the elastic center 70 of the blade cross section(transverse plane at first distance d₁). The first secondary sensor 47Blies on a first secondary sensor axis 76 extending through the elasticcenter 70 of the blade cross section. In FIG. 7, the elastic center lieson the chord 60 and thus the main axis 72 coincide with the chord 60.The angle α₁ between the two sensor axes 74 and 76 is 90°. The angle β₁between the main axis 72 and the first primary sensor axis 74 is 75°.

In FIG. 6 and FIG. 7, the sensors are embedded in the shell body 78 ofthe wind turbine blade.

FIG. 8 is a cross section illustrating an example of sensor setpositioning on a wind turbine blade. The wind turbine blade 10′″comprises a beam 80 attached to the shell body 78 and the second primarysensor 47B is attached to the beam 80 nearest the leading edge 56.

FIG. 9 is a cross section illustrating an example of first sensor setpositioning on a wind turbine blade at a first distance d₁. The windturbine blade comprises a shell body 78 and the sensors 47A, 47B aremounted on the inner surface of the shell body 78. The angle α₁ betweensensor axes 74 and 76 is 90°.

FIG. 10 is a cross section illustrating an example of second sensor setpositioning on a wind turbine blade at a second distance d₂. The windturbine blade comprises a shell body 78 and the sensors 48A, 48B aremounted on the inner surface of the shell body 78. The angle α₂ betweensensor axes 74 and 76 is 90°.

FIG. 11 illustrates a part of a wind turbine. The wind turbine comprisesa hub 8 from which the blades whereof a first wind turbine blade 10 isshown extend substantially in a radial direction when mounted to the hub8. The wind turbine blade 10 comprises a sensor system 82 with anoptical path comprising a first optical fiber 84, a second optical fiber86 and a patch optical fiber 88. Optical connector 90 couples the firstoptical fiber 84 and the patch optical fiber 88 and optical connector90′ couples the second optical fiber 86 to the patch optical fiber 88.The optical fibers 84, 86, 88 are SM1500 (4.2/125) fibers. The firstoptical fiber comprises first primary sensor 47A and second primarysensor 48A in the form of fiber Bragg gratings and optionally firsttemperature sensor 98A. The second optical fiber comprises firstsecondary sensor 47B and second secondary sensor 48B in the form offiber Bragg gratings and optionally second temperature sensor 98B. Thefirst end 85 of the first optical fiber 84 is coupled to a reading unit92 for reading sensor signals from the sensor system 82. The readingunit 92 provides wavelength values of the sensor signals to a blademonitoring system 94 via a data cable 96. The blade monitoring system isconfigured for estimating components of the bending moment at the rootend of the wind turbine blade based on the sensor signals and configuredfor transmitting the estimated bending moment to a turbine controller(not shown). The second end 85′ of the first optical connector 84 isoptically coupled to the first end 89 of the patch optical fiber 88 inconnector or connector assembly 90. The second end 89′ of the patchoptical fiber 88 is optically coupled to the first end 87 of the secondoptical fiber 86 in connector or connector assembly 90′.

FIG. 12 schematically illustrates the optical connectors or connectorassemblies 90 between the first optical fiber 84 and the patch fiber 88.The first optical fiber 84 comprises a first core 130 with a first corediameter d_(core, 1). The patch optical fiber 88 comprises a patch core132 with a patch core diameter d_(core,p)=d_(core,1)=4.2 μm. Fibercladding material and sheet 134, 136 protect the cores 130, 131, 132.The first optical fiber 84 comprises a first end connector part at thefirst end (not shown) and a second end connector part 138 (e.g. femaleE2000 connector) at the second end 85′, and the patch optical fibercomprises a first end connector part 140 (e.g. male E2000 connector) forconnecting the first optical fiber 84 and the patch optical fiber 88.The connector assembly 90′ is formed in the same way as the connectorassembly 90 indicated by the reference numbers.

The second optical fiber 86 includes a second core 131 with a secondcore diameter d_(core,2)=d_(core,p), wherein the second optical fiberextends from a first end to a second end and comprising at least onesensor. The second optical fiber 86 comprises a first end connector part138′ (e.g. female E2000 connector) at the first end 87 and a second endconnector part (not shown) at the second end, and the patch opticalfiber comprises a second end connector part 140′ (e.g. male E2000connector) for connecting the second optical fiber 86 and the patchoptical fiber 88.

FIG. 13 schematically illustrates a blade monitoring system 94. Theblade monitoring system 94 comprises a housing 95 accommodating aprocessing unit 150 connected to an interface 152 and a memory unit 154via connections 155, 155′, respectively. The interface 152 comprises afirst connector port 156 and a second connector port 158. The firstconnector port 156 is configured for connection to a reading unit forreceiving data of sensor signals from a sensor system of a wind turbineblade. The second connector port 158 is configured for connection to aturbine controller for transmitting and/or sending data and/orcontrol/alarm signals to a turbine controller.

The processing unit 150 is configured for receiving a first sensor setsignal indicative of a first bending moment at a first sensor positionof a wind turbine blade extending along a longitudinal axis from a rootend to a tip end via the first connector port 156. Further, theprocessing unit 150 is configured for estimating a bending moment orcomponents thereof at a first estimation position along the longitudinalaxis based on the first sensor set signal, wherein the first sensorposition is different from the first estimation position along thelongitudinal axis.

FIG. 14 schematically illustrates a blade monitoring system 94′ whereina reading unit 92 is integrated in the blade monitoring system andconnected to the processing unit 150 via connection 155″. The interface152 comprises a first connector port 156 in the form of a first sensorport 160 for coupling the sensor system, e.g. the first optical fiber84, (optionally via a patch optical fiber) to the reading unit 92 of theblade monitoring system 94′.

FIG. 15 schematically illustrates estimation of a primary componentM_(X,est,1) of bending moment at the root end (first estimationposition) of a wind turbine blade having L=53.2 m. The first distance d₁is 7 m and the second distance d₂ is 10.5 m. The primary componentM_(X,est,1) is estimated based on a first primary sensor signal S₁₁ froma first primary sensor at d₁ and a second primary sensor signal S₂₁ froma second primary sensor at d₂.

It has been shown that using cubic spline functions from the firstprimary sensor to the tip end of the wind turbine blade and then usingthe gradient or derivative of the bending moment at the first distanceto perform a linear extrapolation from d₁ to the root end may bepreferred. Gradients of the bending moment at the tip end are zero.Further, the bending moment gradient at d₁ may be estimated based on thebending moments at d₁ and d₂, e.g. with a backward Euler method. A largedistance between the first and second sensor sets may not be desirable.

The estimation as illustrated in FIG. 15 comprises the following stepsusing measured bending moment components at d₁ and d₂:

-   -   A first cubic spline function with “correct” boundary conditions        using the two measurement points and the tip end point where the        bending moment is zero is fitted used to perform the        interpolation of a point near the end of the wind turbine blade.        Here “correct” means that the first derivatives at the two ends        of the interval are correct,    -   the bending moment gradient at d₁ is estimated using the        measured sensor data transformed to bending moments at d₁        (M_(X,1)) and d₂ (M_(X,2)) with a backward Euler method, and the        derivative at the tip end is zero. The three points, i.e. d₁, d₂        and the tip end point are denoted points A in FIG. 15. The        interpolated point near the end of the wind turbine blade is        denoted point B.    -   Then a second spline function, covering the interval from d₁ to        the tip end is constructed using a “not-a-knot” method where the        points A and B are used.    -   Then, the second spline function is extended to the root end        with a linear extrapolation denoted app. A′ between d₁ and the        root end.

The “not-a-knot” method means that the third order derivative at thesecond and second last point of the domain used for interpolation is thesame when looking from each side of the point.

FIG. 15 illustrates estimations of the bending moment. In the graph,app. A′ represents sensor positions at d₁=7 m and d₂=10.5 m, and app. B′represents sensor positions at d₁=7 m and d₂=20 m. As can be seen onFIG. 15, app A′ provides a better estimation of primary component nearthe root end while app B′ provides a better estimation between 10 m and50 m from the root end.

It should be noted that in addition to the exemplary embodiments of theinvention shown in the accompanying drawings, the invention may beembodied in different forms and should not be construed as limited tothe embodiments set forth herein. Rather, these embodiments are providedso that this disclosure will be thorough and complete, and will fullyconvey the concept of the invention to those skilled in the art.

LIST OF REFERENCE NUMERALS

-   2 wind turbine-   4 tower-   6 nacelle-   8 hub-   10, 10′, 10″,-   10′″ wind turbine blade-   14 blade tip-   16 blade root-   18 leading edge-   20 trailing edge-   22 pitch axis-   30 root region-   32 transition region-   34 airfoil region-   40 shoulder-   41, 42, 43,-   44, 45, 46 airfoil profile-   47A first primary sensor-   47B first secondary sensor-   48A second primary sensor-   48B second secondary sensor-   50 airfoil profile-   52 pressure side-   54 suction side-   56 leading edge-   58 trailing edge-   60 chord-   62 camber line/median line-   70 elastic center-   72 main axis-   74 primary sensor axis-   76 secondary sensor axis-   78 shell body-   80 beam-   82 sensor system-   84 first optical fiber-   85 first end of first optical fiber-   85′ second end of first optical fiber-   86 second optical fiber-   87 first end of second optical fiber-   87′ second end of second optical fiber-   88 patch optical fiber-   89 first end of patch optical fiber-   89′ second end of patch optical fiber-   90, 90′ optical connector-   92 reading unit-   94 blade monitoring system-   94′ blade monitoring system-   95 housing-   96 data cable-   98A first temperature sensor-   98B second temperature sensor-   99 beam splitting/combining unit-   130 first core-   131 second core-   132 patch core-   134 fiber cladding material and sheet-   136 fiber cladding material and sheet-   138 second end connector part of first optical fiber-   138′ first end connector part of second optical fiber-   140 first end connector part of patch optical fiber-   140′ second end connector part of patch optical fiber-   150 processing unit-   152 interface-   154 memory unit-   155, 155′,155″,-   155′″, 155″″ connection-   156 first connector port-   158 second connector port-   160 sensor port-   c chord length-   d_(t) position of maximum thickness-   d_(f) position of maximum camber-   d_(p) position of maximum pressure side camber-   d_(s) shoulder distance-   d_(root) root diameter-   f camber-   L blade length-   P power output-   r local radius, radial distance from blade root-   t thickness-   v_(w) wind speed-   θ twist, pitch-   Δy prebend-   α₁ angle between first primary sensor axis and first secondary    sensor axis-   α₂ angle between second primary sensor axis and second secondary    sensor axis.-   β₁ angle between first primary sensor axis and main axis-   β₂ angle between second primary sensor axis and main axis-   d_(core,1) first core diameter-   d_(core,2) second core diameter-   d_(core,p) patch core diameter

1. A wind turbine blade extending along a longitudinal axis from a rootend to a tip end and in a transverse plane perpendicular to thelongitudinal axis, the transverse plane having a main axis extendingthrough an elastic center point the wind turbine blade comprising ablade shell forming a profiled contour including a pressure side and asuction side, as well as a leading edge and a trailing edge with a chordhaving a chord length extending there between, the main axis beingparallel to the chord, wherein the profiled contour comprises a rootregion, an airfoil region and optionally a transition region, andwherein the wind turbine blade comprises a sensor system including afirst sensor set for measuring a first bending moment in a first sensorposition at a first distance from the root end, the first sensor setcomprising a first primary sensor for measuring a primary component anda first secondary sensor for measuring a secondary component, the firstsensor set being arranged in the transition region or the airfoil regionof the blade, wherein a first primary sensor axis in the transverseplane is oriented in a direction defined by the first primary sensor andthe elastic center point, and a first secondary sensor axis in thetransverse plane is oriented in a direction defined by the firstsecondary sensor and the elastic center point, and wherein an anglebetween the first primary sensor axis and the first secondary sensoraxis is in the range from 50° to 130°.
 2. A wind turbine blade accordingto claim 1, wherein the first sensor set only has a first primary sensorand a first secondary sensor for measuring the first bending moment. 3.A wind turbine blade according to claim 1, wherein the angle between thefirst primary sensor axis and the first secondary sensor axis is in therange from 85° to 95°.
 4. A wind turbine blade according to claim 3,wherein the angle between the first primary sensor axis and the firstsecondary sensor axis is 90°.
 5. A wind turbine blade according to claim1, wherein the first primary sensor is positioned on the pressure sideof the wind turbine blade.
 6. A wind turbine blade according to claim 1,wherein the first secondary sensor is positioned at the leading edge ofthe wind turbine blade.
 7. A wind turbine blade according to claim 1,wherein the wind turbine blade comprises a second sensor set formeasuring a second bending moment in a second sensor position at asecond distance from the root end, the second sensor set comprising asecond primary sensor for measuring a primary component and a secondsecondary sensor for measuring a secondary component, wherein a secondprimary sensor axis in the transverse plane is oriented in a directiondefined by the second primary sensor and the elastic center point, and asecond secondary sensor axis in the transverse plane is oriented in adirection defined by the second secondary sensor and the elastic centerpoint, and wherein an angle between the second primary sensor axis andthe second secondary sensor axis is in the range from 50° to 130°.
 8. Awind turbine blade according to claim 1, wherein the first distance isequal to at least a diameter of the root.
 9. A wind turbine bladeaccording to claim 1, wherein the primary and secondary sensors arestrain gauge sensors.
 10. A wind turbine blade according to claim 1,wherein the primary and secondary sensors are optical sensors, such asfiber Bragg gratings.
 11. A wind turbine blade according to claim 1,wherein at least one of the sensors is embedded in the blade shell. 12.A wind turbine blade according to claim 1, wherein the wind turbineblade comprises a beam attached to the shell and wherein at least one ofthe sensors is mounted on the beam.
 13. A wind turbine blade accordingto claim 1, wherein the wind turbine blade comprises a web attached tothe shell and wherein at least one of the sensors is mounted on the web.14. A wind turbine blade according to claim 1, wherein the length of theblade is at least 40 m.